Following Max Delbruck's advice, Dulbecco visited the major centers of animal virus work in the US in order to discover a way to quantitatively assay animal viruses by a plaque technique, similar to the technique that had recently been developed for bacterial viruses (bacteriophages). Within less than a year Dulbecco worked out such a method for Western equine encephalitis virus, which then opened up animal virology to quantitative work. The technique was then used by Dulbecco and Vogt to study the biological properties of poliovirus. Digital facsimile from PubMedCentral at this link.